The present invention relates to a continuous process for the preparation of high-quality imidazoline compounds by the reaction of a diamine and a higher fatty acid.
Imidazoline is a compound insoluble in water but dispersible in water giving a homogeneous aqueous dispersion exhibiting excellent foaming. On the other side, derivatives of imidazoline are generally soluble in water and their aqueous solution is useful as an amphoteric surface active agent exhibiting excellent detergency and foaming with an advantage of very low irritation to human skins. Therefore they can find very wide applications as a detergent ingredient in shampoo and various kinds of detergents for kitchen and laundry use as well as an emulsifying agent or a basic ingredient in cosmetics. Thus the demand for these imidazoline compounds is rapidly growing and an effective method for the preparation of these compounds with high quality is earnestly desired.
In the prior art, several methods are proposed for the preparation of the imidazoline compounds by the reaction of a diamine and a higher fatty acid. See, for example, British Pat. No. 985,321 and U.S. Pat. No. 3,408,361. The method disclosed in the former reference, in which the starting reactants are brought into reaction under agitation in a vessel with introduction of nitrogen gas while the distillate is continuously removed out of the vessel, is, however, defective due to the very low maximum yield of the desired product.
In the method disclosed in the latter reference, the reaction is carried out in a vessel, of which the temperature and the pressure are gradually changed, with continuous addition of the diamine at a rate to compensate the amount distilling out of the vessel taking several hours to complete the reaction. This method is advantageous by the high maximum yield reached but has problems in that (a) the control operation of the temperature and the pressure throughout the reaction time is very complicated so that hardly no reproducibility of the results can be expected from the standpoint of practice, (b) practical difficulty is involved in the control of the rate of continuous introduction of the diamine in balance with the amount distilled out, and (c) re-use of the distilled and recycled diamine as a reactant in the reaction is not recommendable because of the variety in the by-products contained therein bringing about a problem of waste disposal. This method is also disadvantaged by the difficulty when modification to a continuous process is intended because of the necessity of the complicated control operation during several hours of the reaction.
Thus very strict control of the parameters in the reaction is required in the prior art with complicated control operation in order to attain high yield of the objective compound because of the extremely low reaction velocity of the imidazolination reaction itself necessitating a batch-wise operation in carrying out the reaction.